State of the Art
To demodulate direct sequence spread spectrum transmissions, chip code phase ambiguities and carrier center frequency ambiguities must be resolved. The frequency ambiguities are caused by an imperfect match of transmitter frequency reference to that of the receiver. If the frequency uncertainty is sequentially searched and the chip code phase position is sequentially searched, then the total acquisition time is the product of the two search processes. This can lead to excessively long search times and inefficient transmitter preamble lengths. One method to overcome this added search time is to widen the receiver's Intermediate Frequency (IF) bandwidth to that of the data bandwidth plus the total system frequency uncertainty. This has the undesirable effect of raising the minimum detectable signal level as well as lowering the process gain achievable.
Conventional, non-spread spectrum, radio systems must contend with frequency imperfection between receiver and transmitter. As more expensive frequency references are utilized, the error term between the transmitter and receiver may be minimized over a greater and greater operating temperature range. Even so, any frequency error must be compensated at the receiver. One conventional means for compensating for frequency error is to widen the IF bandwidth to be somewhat greater than that of the bandwidth of the data being received. This technique of widening the IF bandwidth has the negative effect of reducing the signal to noise ratio available. As an alternative, the full signal-to-noise ratio may be recovered but at the penalty of added acquisition time. These techniques may use frequency lock loops with wide locking ranges or may scan over a range of frequencies equal to the total system frequency uncertainty.
In a direct sequence spread spectrum system, there is the frequency uncertainty inherent in a conventional radio system, as well as the time uncertainty of the chip code phase position between the transmitted signal and the receiver chip code reference. If the transmitter performs a frequency search, as well as a chip code alignment search, then the total search time is the product of the two processes. Alternatively, the direct sequence receiver may widen its last IF bandwidth to be greater than the data modulation being received. This again causes a reduction in achievable receiver sensitivity.
A further drawback in direct sequence serial acquisition receiving detectors is that there is little benefit to searching at a wide bandwidth and then, upon some initial Trip condition initiated by proper chip code alignment, further reducing the IF bandwidth in a second step. The difficulty in such a technique is that the effective signal to noise ratio must be adequate to provide the initial Trip. The effect is that many Trips will be lost when the system is operating at its minimum detectable signal level.
Further exacerbating the problems with direct sequence serial acquisition receiving detectors, is the fact that many direct sequence transmission systems operate in a packet mode. The packet mode operation is desirable for reducing the transmitter turn-on time and, therefore for increasing battery life, or to enhance the aloha collision performance between multiple unsynchronized transmitters, or as a tactic to increase the overall message traffic throughput. In any of these cases, it is desirable to utilize a preamble for spread spectrum acquisition which is as short as possible.
An alternative to serial correlation search techniques are various parallel correlation methods. These methods also suffer from frequency uncertainty between the transmitter's crystal reference and that of the receiver. Even with the added expense and complexity of parallel correlation methods, additional techniques must be utilized to compensate for frequency error, which are similar in nature as those employed by conventional non-spread spectrum radio systems.
Dixon's textbook on spread spectrum communications revision 3 teaches the use of carrier tracking in order to resolve the uncertainty between a transmitted signal and a receiver's frequency reference. Dixon further teaches the use of phase lock loops to track the frequency uncertainty. Dixon does not teach the use of parallel IF filter banks or the like. In U.S. Pat. No. 4,977,577, Arthur et al, direct sequence spread spectrum acquisition techniques are taught for serial correlation architectures. Arthur et al, teaches the use of a last IF filter bandwidth which is large enough to compensate for the frequency uncertainty between the transmitter's reference and the receiver's reference. Arthur et al, does not teach the use of parallel IF filter banks in order to enhance the sensitivity achieved by the receiver. Hillier (U.S. Pat. No. 4,864,588) teaches the use of parallel correlating devices to obtain synchronization of a received direct sequence spread spectrum modulated signal. Hillier transmits a special square-wave modulation in order to enhance signal acquisition time. Hillier does not teach the use of parallel filter banks in the receiver to enhance sensitivity of the receiver.